1. Field of the Invention
This invention relates generally to a corona discharge igniter for receiving a voltage from a power source and emitting an electrical field for ionizing and igniting a mixture of fuel and air of an internal combustion engine, and methods of manufacturing the same.
2. Description of Related Art
Igniters of corona discharge air/fuel ignition systems include an electrode received in an insulator and extending longitudinally from an electrode terminal end to an electrode firing end. The electrode terminal end receives a voltage from a power source and the firing end emits an electrical field to ionize and ignite a mixture of fuel and air in a combustion chamber. The electrode typically includes a corona enhancing tip at the firing end, as shown in prior art FIG. 2, for emitting the electrical field. The electrical field includes at least one streamer, and typically a plurality of streamers that form a corona. The corona igniter does not include any grounded electrode element in close proximity to the corona enhancing tip. Rather, the mixture of air and fuel is ignited along the entire length of the high electrical field generated from the corona enhancing tip.
The corona enhancing tip is typically formed of a base material including nickel. The corona enhancing tip typically includes branches each extending from a platform to a distal end, as shown in FIGS. 2-2B. The corona enhancing tip includes an exposed outer surface presenting radius features, such as spherical radii, along the edges and at the distal ends of each branch. As shown in FIGS. 2-2B, the electrical field emitted by the corona enhancing tip is concentrated at the sharpest point or points of the exposed outer surface, i.e. the smallest radius feature or spherical radius. As shown in FIG. 22, the smaller the spherical radius, the stronger the electrical field emitted by the corona enhancing tip. The corona enhancing tip also has a diameter extending between opposite distal ends. As shown in FIG. 23, the diameter of the corona enhancing tip is directly related to the strength of the electrical field.
As shown in FIGS. 2, 2A, and 2B, the corona enhancing tip is typically designed to include the smallest spherical radii at the distal ends of the branches so that the electric field is concentrated and of sufficient strength. However, during use of the electrode in the internal combustion engine, the voltage received by the corona enhancing tip over time causes electrical erosion of the corona enhancing tip. In addition, the corona enhancing tip experiences oxidation or chemical corrosion due to the extreme temperatures, pressures, and constituents of the combustion chamber. As shown in FIGS. 3, 3A, and 3B, the electrical erosion and chemical corrosion causes the corona enhancing tip to experience a reduction in volume. The spherical radius at the distal ends increases and the diameter of the corona enhancing tip decreases. FIGS. 20 and 21 illustrate how the spherical radius of the conventional corona enhancing tip can increase over time due to the erosion and corrosion. Thus, the strength of the electrical field emitted from the corona enhancing tip decreases and the ignition performance degrades. Further, over time, the spherical radius of the distal ends may become greater than a spherical radius located between the corona enhancing tip and the insulator, the electric field to be emitted from the wrong point or an irregular ignition position, as shown in FIG. 3, and referred to as arcing, which is undesirable in many situations. The arcing and/or irregular ignition position also degrades the quality of ignition of the air-fuel mixture.